A colourless gas, NO; m.p. –163.6°C; b.p. –151.8°C. It is soluble in water, ethanol, and ether. In the liquid state nitrogen monoxide is blue in colour (r.d. 1.26). It is formed in many reactions involving the reduction of nitric acid, but more convenient reactions for the preparation of reasonably pure NO are reactions of sodium nitrite, sulphuric acid, and either sodium iodide or iron(II) sulphate. Nitrogen monoxide reacts readily with oxygen to give nitrogen dioxide and with the halogens to give the nitrosyl halides XNO (X = F,Cl,Br). It is oxidized to nitric acid by strong oxidizing agents and reduced to dinitrogen oxide by reducing agents. The molecule has one unpaired electron, which accounts for its paramagnetism and for the blue colour in the liquid state. This electron is relatively easily removed to give the nitrosyl ion NO⁺, which is the ion present in such compounds as NOClO₄, NOBF₄, NOFeCl₄, (NO)₂PtCl₆ and a ligand in complexes, such as Co(CO)₃NO.

In mammals and other vertebrates, nitrogen monoxide is now known to play several important roles. For example, it acts as a gaseous mediator in producing such responses as dilation of blood vessels, relaxation of smooth muscle, and inhibition of platelet aggregation. In certain cells of the immune system it is converted to the peroxynitrite ion (⁻O–O–N=O), which has activity against tumour cells and pathogens.